This invention relates to a combined data base and transaction system comprising a communications module at a central location and a plurality of remote subscriber units that communicate with the communications module over a common-carrier transmission medium such as a telephone system. More particularly, it relates to a system employing conventional tone-dialing pads to generate the signals transmitted from the subscriber units to the communications module and employing novel techniques in the transmission of data from the communications module to provide high-quality displays at the subscriber units.
The invention has particular utility in so-called "videotex" systems, in which subscribers can gain access to large central data bases from subscriber units located, for example, in the subscribers' homes and also enter into various business transactions generally related to the contents of the data bases. Typically a transcriber dials up a communications module over a telephone line and receives a response in the form of a display on a video screen. The response typically lists a "menu" of functional options open to the subscriber, and the subscriber in turn responds with a designation of his selection. The communications module then transmits a new display containing information and/or additional selections within the selected category. This interchange continues until the subscriber's video display has narrowed down to a particular item of information he wishes to obtain or a transaction in which he wishes to engage. The present invention is directed to a reduction in the cost of the subscriber units so as to make this type of service more widely acceptable.
In accordance with the invention, transmissions from the communications module to the subscriber units are preferred by means of conventional digital modulation techniques. For example, frequency-shift keying may be employed, with the data transmitted in blocks, each of which also contains error-detection coding. Transmissions from the subscribers to the communications module, on the other hand, are in the form of DTMF signals such as those generated by a conventional tone-dialing key pad. Thus, the subscriber can use a tone-dialing pad instead of a more expensive keyboard and can, in fact, use the keys that are incorporated in his telephone set.
This does not, of course, permit the transmission of a full set of alphanumeric characters to the communications module. However, the ten decimal digits are fully sufficient, since they can be used to designate selections of various options displayed on a screen in response to transmissions from the communications module to the subscriber unit. The two additional keys on the tone-dialing pad (* and #) can serve as function keys, one of them, for example, providing an "enter" function, signaling the communications module to act in response to the key or keys actuated before the function key. The invention can be employed, however, with a full alphanumeric keyboard. The extra frequency combinations employed in the extra keys of a conventional sixteen-key pad can be used to indicate that the combination is part of a sequence of two-frequency combinations that together encode for characters from the larger set of alphanumeric characters.
The subscriber unit also includes a display unit, such as a cathode-ray tube (CRT), which can be the subscriber's own television receiver. Finally, as the only device that must be supplied to the subscriber to enable him to use the service, the subscriber unit includes a small housing containing (1) a sensor that picks up incoming signals from the communications module as received in the telephone handset, (2) a receiver that receives and decodes the signals from the communications module and (3) display circuitry for displaying received data on the CRT screen. The display circuitry includes a random-access refresh memory having a set of storage locations corresponding to locations on the CRT screen and a character generator that responds to the contents of successive storage locations by providing display signals that generate corresponding characters in the associated screen locations.
The refresh memory, character generator, and associated control circuitry are conventional and of relatively low cost. In this connection, I note that in the preferred embodiment of the invention the characters that can be displayed include not only the usual alphanumeric and punctuation characters but also a set of graphic characters that, when displayed in sets of adjacent screen locations, can provide graphic displays. Again, this is a conventional technique for generating graphic displays.
The communications module transmits both character-identifying codes and control codes to the subscriber unit. The control codes include, for example, codes representing the addresses in the refresh memory in which succeeding character-identifying codes are to be stored or the location on the display screen in which a cursor is to be displayed. The subscriber unit includes circuitry for decoding and responding to these and other control codes so as to store incoming character-identifying codes in the appropriate storage locations and to perform various control functions such as blinking the cursor at the appropriate screen location.
In accordance with conventional data-transmission techniques, the communications module transmits data to each subscriber unit in blocks, each block including an error-detection code for the data in the block. According to the present invention, each data block is self-contained in that it contains all of the control codes required for storage of the character-identifying codes in the block and for display of the characters corresponding with those character-identifying codes. Thus, a subscriber unit's use or display of data contained in a data block does not depend on the reception of any other data block.
The communications module continuously transmits display information to each subscriber unit except when it detects a DTMF signal from a subscriber unit. When such a signal is detected, the communications module interrupts its transmission to that subscriber unit until the DTMF signal terminates.
In the usual course of events the communications module transmits an entire display at least several times before the display must be changed in response to a signal received from the subscriber. At the subscriber's end of the system, whenever the subscriber unit detects an error in a data block received from the communications module, it ignores the entire block. The contents of the corresponding locations in the refresh memory are thus unchanged, and the corresponding screen locations therefore display data previously transmitted by the communications module. For several reasons, this data-transmission arrangement minimizes the visual effect of the resulting gaps in the received data.
In the first place, these transmission errors are generally due to noise, which is largely random in nature and therefore is unlikely to affect the same portion of the screen on the next transmission of the data. Therefore, the gaps are generally filled in on the next pass. Moreover, since errors in transmission result in no change in the display in the corresponding screen locations, they do not degrade those portions of the display already containing the correct display characters. Thus, successive passes through the display correct the incorrect portions of the display and, after a few such passes, the entire display faithfully reproduces the depiction sent by the communications module.
Generally, the communications module does not transmit all parts of the display with the same frequency. For example, if only a part of the display is changed, the communications module will for a time transmit that part more often than it transmits unchanged portions of the display. This increases the speed with which new portions of the display attain an error-free condition. Furthermore, as described below, there are various other conditions under which the communications module will refresh different portions of a subscriber's display at different rates.